This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The Loh lab specializes in designing proteins to make useful molecular devices or to answer fundamental questions about the thermodynamics and kinetics of protein folding. In some cases it is necessary to determine x-ray structures of the resulting proteins in one or more states to confirm the anticipated structures. The in-house source is not working yet, so we don't have unit cell/space-group/resolution info. We propose to solve the X-ray structures of two classes of engineered protein switches. The first is a two-domain fusion protein which can exist either as a monomer or as a domain-swapped dimer. These molecules are being used to create homotypic protein-protein binding interfaces. The second class is being developed as optical biosensors. These proteins are engineered to undergo a specific conformational change upon binding to ligand. X-ray diffraction will be used to characterize the structures of these molecular switches in their